It makes me proud that the great Matthew Weier O’Phinney 1 now describes the same technique to test code that emits output, especially code that sends headers with the built-in header() function. Read more in his post here:

In my opinion this is a great example of how useful this method is. “Headers already sent” errors in your unit tests can drive you crazy. Unfortunately, there are still many applications that do not use namespaces (*cough* Magento *cough*) where it does not work.